The generation of noise from an engine system used for a motor vehicle is caused by the engine system itself, a piston system or an intake system. Among them, noise from the intake system is generated coming from the difference between the inner pressure and the outer pressure of a combustion chamber when an intake valve being opened or closed. It is transmitted therefrom to an air cleaner and then dispersed toward the outside.
The intake noise is influenced by the frequency, generally the low frequency, related with the revolution number of an engine.
As well-known, noise generated due to the low frequency should be eliminated if possible since it resonates a vehicle body itself to produce noise inside the motor vehicle.
Kawamura's U.S. Pat. No. 4,136,756 provides a suction air muffler for motorcycle effective to prevent producing noise which would otherwise be produced by air drawn by suction into the inlet of the motorcycle. Takai's U.S. Pat. No. 4,350,223 provides a silencer for reducing noise produced by air drawn into the engine such that a resonance chamber is placed on a conduct connected to an air cleaner and porous metal tubes connect the space inside the chamber to the conduct. Hanzawa et al U.S. Pat. No. 4,800,985 proposes an intake air silencer including a side branch tube which is flexible at least partially for installation in a limited space, placed on an intake tube connected to an air cleaner to reduce noise.
As described above, many kinds of silencers for reducing the air intaking noise into the engine have been commercially used. Generally, they are divided into two types. One is a side branch type silencer, which can reduce noise by virtue of the mutually interfering of sonic waves according to quarter wave filter theory. Another is a cavity type silencer, which can silence noise by changing a wave energy of a sonic wave into a going and coming energy at the inside of a resonance chamber.
The present invention is related with an improvement of the side branch type silencer. FIG. 5 shows a basic structure of the side branch type silencer. The resonance chamber B is formed on an air intake tube in the form of connecting with each other, so that, when the air intaking noise arrives at the inside of the intake tube, it resonates the resonance chamber B. The mutual interference between resonating wave and noise occurs therein thereby reducing noise.
The engine provided with the silencer of this type is known to obtain the improved horse power and fuel consumption ratio as a graph shown in FIG. 6 since the path of the intake tube with a limited space has the sufficient amount of existed therein, air thereby causing the high charging efficiency of air drawn into the inside of a cylinder. The theoretical reduction of noise frequency of the side branch type silencer can represent F=C/4L as a formula, C is an acoustic velocity and L is the length of the resonance chamber. This formula means that the effective length of the resonance chamber can be variable to widen the reducible low frequency region. Thus, if the length or the capacity of the resonance path varies with the change of the RPM sensor of the engine, the preferred efficiency of noise reduction can be obtained.
Japanese Specification No. 86-41815 proposes the silencer of the variable type such that some short and long pipes connect the path of the intake tube with the resonance box and entrances of the pipes are provided with the opened and closed valves. Each opened and closed valve gears with the revolution of the engine thereby forming the simple structure and effectively reducing noise independently of the change of the engine revolution. However, the pipes interposed between the intake path and the resonance box can not help having a large size unfit for the limited engine chamber. Japanese Specification No. 61-41813 provides the silencer such that a piston placed at the inside of the resonance tube slides by a stepping motor corresponding to the engine revolution thereby increasing and/or decreasing the cavity of the resonance chamber, so that such problem can be solved. The invention has the advantage of minimizing the size of the silencer. However, since a tool of the piston type slides riding on a screw rod rotated by the stepping motor, the complex structure can be formed. Especially, the piston can not be rapidly changed immediately when the revolution number of the engine is changed thereby causing the poor efficiency of noise reduction though in a moment.